Estrogen, a female sex hormone, through binding to its cognate Estrogen receptors, ERα and ERβ, governs a wide range of physiological processes, e.g., the development of the female reproductive system, the maintenance of bone mass, and the protection of cardiovascular tissue and the central nervous system. Upon estrogen's binding to an estrogen receptor (“ER”), the receptor undergoes a conformational change resulting in its homodimerization. The ER homodimer then binds to estrogen-response elements (“EREs”) that are present in the promoters of a specific set of target genes and regulates their expression with the help of transcriptional coregulators. Several thousand canonical ER target genes have been identified, many of which regulate cell proliferation and survival.
Because ER signaling is implicated in many pathways, it is well known that deregulation of ER signaling, specifically through ERα, results in uncontrolled cellular proliferation which eventually results into cancer. ER+ breast cancer accounts for approximately 75% of all breast cancers diagnosed, as well as some ovarian and endometrial cancers. The prevalence of ER+ cancer has led to decades of investigation and development of antiestrogens as therapeutic agents.
Antiestrogen (i.e., hormonal) therapy is the first choice for treatment of most ER+ breast cancers. There are three major classes of antiestrogen therapies, including aromatase inhibitors (e.g., letrozole and anastrozole); selective estrogen receptor modulators (e.g., tamoxifen, toremifene, and raloxifene); and selective estrogen receptor degraders (e.g., fulvestrant). These classes of antiestrogen therapy operate by different mechanisms of action, such as inhibiting aromatase enzyme, competitively binding to ERα, and/or causing ERα degradation.
The aforementioned therapies may result in deleterious effects. For example, administration of aromatase inhibitors results in a decrease in bone mineral density, which can result in an increased risk of fractures. Administration of selective estrogen modulators can result in development of endometrial cancer and/or cardiovascular issues, e.g., deep thrombosis and pulmonary embolism. Additionally, the aforementioned therapies may suffer from insufficient clinical efficacy.
Accordingly, there exists a need to treat ER+ cancer without the harmful side effects known for current therapies. One approach to achieve this goal would be to utilize the naturally occurring cellular ubiquitin-mediated degradation. Without being bound to any theory, it is believed that ERα degradation may occur when both ERα and a ubiquitin ligase are bound and brought into close proximity.
Cereblon (“CRBN”) E3 ubiquitin ligase is a ubiquitin ligase that CRBN forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 and Cullin 4. It functions as a substrate receptor by bringing the substrates to close proximity for ubiquitination and subsequent degradation by proteasomes. Recently, it has been discovered that small molecules drugs, e.g., thalidomide and its close analogs, lenalidomide and pomalidomide, can simultaneously interact with CRBN and some other proteins. In doing so, CRBN may be exploited for target protein degradation, such as IKZF1 and IKZF3. This is thought to account for the anti-myeloma effects of thalidomide and related compounds.